Fertilizers are responsible for over half of global food production, but there are areas in world with nutrient deficiency and other areas of nutrient excess.
Managing mineral plant nutrients requires careful application of science and skill to meet production, environmental, and social goals.

Monday, January 28, 2013

Nitrogen Sources for Organic Crop Production

Nitrogen is generally
the most difficult nutrient to manage for organic crop production. Cover crops
and composts can contribute substantial N for crops, but it is challenging to
synchronize N release from these materials with the plant demand. Various
commercial organic N fertilizers are available, but their costs may be
prohibitive in many situations. Careful management of organic N sources is
required to meet crop requirements, while avoiding undesirable N losses to the
environment. This article was first written by me and Dr. Tim Hartz (Univ. Calif, Davis) for the magazineBetter Crops

Nitrogen is the plant nutrient that is often most
limiting to efficient and profitable crop production. Inadequate supply of
available N frequently results in plants that have slow growth, depressed
protein levels, poor yield of low quality produce, and inefficient water use.
Nitrogen-stressed plants often have greater disease susceptibility compared
with properly nourished plants. However, excessive N can be detrimental for
crop growth and quality, in addition to causing undesirable environmental
impacts. For these reasons, more research has been conducted on managing this
plant nutrient than any other. This brief review does not address all the
important aspects related to N management, but covers the major sources of N
for organic crop production and their behavior in soil. An extensive list of
references is available at this website:

Although Earth’s atmosphere contains 78% N gas (N2), most organisms cannot directly use this resource due to the stability
of the compound. Breaking the strong chemical bond in N2
gas requires either the input of energy (to manufacture fertilizer) or
specialized nitrogenase enzymes. Since the use of manufactured N fertilizer is
not allowed for organic production, these materials are not specifically
addressed here.

The nitrogen cycle

There
are many biological and chemical processes that cause first-year recovery by
plants to generally be less than 50% of the applied N. Low N efficiency can
also be caused by imbalance of other essential plant nutrients. Management of N
is also made difficult due to uncertainties related to weather events following
fertilization. Where N recovery is low, it is important to consider where the
unrecovered N may be going and the potential environmental and economic risks
associated with these losses (Figure 1).

Almost
all non-legume plants obtain N from the soil in the form of ammonium (NH4+) or nitrate (NO3-). Some organic
N-containing compounds can be acquired by roots in small amounts, but these are
not a major source of plant nutrition. Ammonium is the preferred inorganic
source of N for some plants (especially grasses), but nitrification processes
typically oxidize this N form to NO3-. Many other crops grow best with predominantly
NO3- nutrition. In most warm,
well-aerated soils, the NO3- concentration
may be at least 10 times greater than the NH4+ concentration.

Unlike
other plant nutrients (like P and K), there is no universal or widely used soil
test to predict the amount of supplemental N required to meet the crop’s need.
Instead, the need for N supplementation is typically based on yield
expectations, field history, and measurement of residual NO3-. Nutrients in
commercial fertilizers are generally soluble, so their availability to plants
is quite predictable. However, most organic N sources require mineralization
(conversion to inorganic forms) before they can be used by plants.
Environmental factors such as soil temperature, pH, moisture, and management
practices such as tillage intensity all impact the rate of N availability from
organic sources.

A
major factor for using organic N sources involves knowing both the amount of N
applied and the rate of N release from the organic material. Nitrogen
availability coefficients are used toestimate
the fraction of total N that will be available for crop uptake during the first growing season (called plant-available N or PAN). The N availability
coefficient can vary widely, based on the nature of the material, management
practices (such as placement), and environmental factors (such as season of the
year). Examples of PAN coefficients are shown in Table 1. Major processes of
the N cycle are described in Table 2.

Mineralization of Organic Matter

When
the crop’s N supply comes exclusively from sources such as soil organic matter,
cover crops, and composts, a thorough understanding of mineralization is
essential to avoid adeficiency
or surplus of available N. Mineralization is not consistent through the year
and crop N demand should be matched with nutrient release from mineralization.
Mineralization rates are dependent on environmental factors (such as
temperature and soil moisture), the properties of the organic material (such as
C:N ratio, lignin content), and placement of the material. Many excellent references
discuss this process in detail. Failure to synchronize N mineralization with
crop uptake can lead to plant nutrient deficiencies, excessive soil N beyond
the growing season, and the potential for excessive NO3– leaching (Figure 2).

Synchronizing nutrient release with plant demand is a challenge with organic materials. Rapid release from organic sources with a low C:N ratio may supply nutrients more rapidly than the plant's demand (A). An organic material with a high C:N ratio may not release nutrients sufficiently rapid to meet the needs of growing plants (B)

Composts: Generally, composts
contain relatively low concentrations of N, P, and K. They typically decompose
slowly and behave as a slow-release source of N over many months or years since
the rapidly decomposable compounds have been previously degraded during the composting
process. Composts can be made from on-farm materials, but they are also widely
available from municipal and commercial sources.

These
composts vary in quality and tend to have low immediate nutritional value, but
provide valuable sources of stable organic matter. Since plastic, trash, and
industrial waste may also turn up in selected municipal composts, some organic
certification programs do not allow their use. Commercially composted manure is
widely available from a variety of primary organic materials.

Manure: The chemical, physical,
and biological properties of fresh manure vary tremendously due to specific
animal feeding and manure management practices. The manure N is present in both
organic and inorganic forms. Nitrogen is unstable in fresh manure because
ammonia (NH3) can be readily lost
through volatilization. Application of fresh manure or slurry on the soil
surface can result in volatilization losses as high as 50% of the total N in
some situations. The combination of wet organic matter and NO3- in some manure can also
facilitate significant denitrification losses. The organic N-containing
compounds in manure become available for plant uptake following mineralization
by soil microorganisms, while the inorganic N fraction is immediately available.
Figure 3 shows the wide range in N mineralization of manure applied to soil.

Determining
the correct application rate of manure and compost to supply adequate PAN
during the growing season can be diffi cult. Begin by having manures and
composts regularly analyzed for nutrient content since there is considerable
variability. The PAN will always be smaller than the total N in the manure
since some loss occurs through volatilization with spreading, and only a
portion of the organic N will be available to the plants during the growing
season following application. The remaining organic N will slowly mineralize in
later years.

When
manures and composts are applied at the rate to meet the N requirement of
crops, the amount of P and K added is generally in excess of plant requirement.
Over time, P can build up to concentrations that can pose an environmental risk
since runoff from P-enriched fields can stimulate the growth of undesirable
organisms in surface water. Excessive soil K can cause nutrient imbalances,
especially in forages. The long-term use of P and K-enriched manures to provide
the major source of N must be monitored to avoid these problems.

Manure spreader in the field

Manures
and composts can be challenging to uniformly apply to the field due to their
bulky nature and inherent variability. Application of raw manure may bring up
concerns related to food safety, such as potential pathogens, hormones, and
medications. The use of raw manure is restricted for some organic uses and
growers should check with the certifying agency before using.

Cover Crops: A wide variety of plant
species (most commonly grasses and legumes) are planted during the period
between cash crops or in the inter-row space in orchards and

vineyards.
They can help reduce soil erosion, reduce soil NO3- leaching, and contribute organic matter and
nutrients to subsequent crops after they decompose. Leguminous cover crops will
also supply additional N through biological N2fixation. The amount of N contained in a cover
crop depends on the plant species, the stage of growth, soil factors, and the
effectiveness of the rhizobial association. Leguminous cover crops commonly
contain between 50 and 200 lb N/A in their biomass.

Cover
crops require mineralization before N becomes plant available. The rate of N
mineralization is determined by a variety of factors, including the composition
of the crop (such as the C:N ratio and lignin content) and the environment
(such as the soil temperature and moisture). As with other organic N sources,
it can be a challenge to match the N mineralization from the cover crop to the
nutritional requirement of the cash crop. It is sometimes necessary to add
supplemental N to crops following cover crops to prevent temporary N
deficiency.

Commercial Organic Fertilizers

Alfalfa pellets can be used as a nitrogen fertilizer

Plant
Products

Alfalfa meal (4% N), cottonseed meal (6% N), corn gluten (9% N), and soybean meal (7% N) are all examples of
plant products that are sometimes used as N sources for organic production.
These products are also used as protein-rich animal feeds. They require
microbial mineralization before the N is available for crop uptake.
Mineralization of these N-rich materials is generally rapid.

is
completely soluble and suitable for distribution through irrigation systems.

Guano: Seabird guano (8 to
12% N) is derived from natural deposits of excrement and remains of birds
living along extremely arid sea coasts. Guano was historically a very important
N source before industrial processes for making fertilizer were developed. Many
of the major guano deposits are now exhausted. Guano is also harvested from
caves where large bat populations roost. It can be applied directly to soil or
dissolved in water to make a liquid fertilizer.

This bat guano contains 10 percent nitrogen

Feather Meal: Feather meal (14 to 16%
N), a by- product of the poultry industry, contains as much as 70 to 90%
protein. It is mostly present as non-soluble keratin stabilized by highly
resistant disulfide bonds. When treated with pressurized steam and
animal-derived enzymes, the feather-based protein becomes a good source of
available N for crop nutrition.Much
of the feather N is not initially soluble, but it mineralizes relatively
quickly under conditions favorable for plant growth.

Pelletizing
the feather meal makes handling and application more convenient. Unprocessed
feathers usually have a delayed N release, but can also be an excellent N
source if the difficulty in uniformly applying low density feathers to the soil
can be overcome.

Fish Meal and Fish
Emulsion:
Non-edible fish (such as menhaden) are cooked and pressed to separate the solid
and liquid fractions. The solids are used as fish meal (10 to 14% N) for
fertilizer and animal feed. The valuable fish oil is removed from the liquid
fraction and the remaining solution is thickened into fish emulsion (2 to 5%
N). Additional processingis
often performed to prevent premature decomposition. The odor from fish meal
products may be unpleasant in a closed environment such as a greenhouse.
Mineralization offish-based
products is generally rapid. Fish products that are fortified with urea to
boost the N concentration are not allowed for organic production.

These
high-N animal byproducts have relatively rapid N mineralization. At typical
summer soil temperatures, more than half of the organic N may mineralize within
2 weeks of application (Figure 4).

Nitrogen mineralization of four common organic N fertilizers at four soil temperatures. Mineralization of N expressed as percent of added organic N (Hartz and Johnstone, 2006).

Seaweed Fertilizers

Seaweed-based
products are typically derived from kelp species (Ascophyllum). Dried kelp
contains approximately 1% N and 2% K, with small amounts of other plant
nutrients.

Due
to their low nutritive content, kelp products are generally used in high-value
cropping situations where economics may be favorable,
or for reasons other than plant nutrition.

Sodium Nitrate

Sodium nitrate (NaNO3, 16% N) is mined from naturally
occurring deposits in Chile and Peru, the location of the driest desert on
earth where NO3 - salts accumulate over time. Sodium nitrate is generally granulated
and readily soluble when added to soil. The intended use of NaNO3in organic
agriculture is typically to meet the N demand during critical plant growth
stages and not to meet the entire nutritional need of the crop. In the U.S.A.,
the use of NaNO3is limited to no more than 20% of the crop N requirement. In some
countries, the use of NaNO3is restricted.

Summing Up

Choosing the “best” source of N for organic crop production is
difficult since nutrient ratios, PAN, mineralization rates, local access, ease
of application, and cost all need to be considered. Computer-based tools are
available to help with these choices. For example, Oregon State University has
an “Organic Fertilizer Calculator” program that allows comparison of various
materials to best meet the fertility needs of a soil. Similar programs are also
available elsewhere.

Each organic N source has unique characteristics that require
special management to gain the most benefit for plant health and economic
production, while minimizing undesirable environmental losses. Commercial
organic sources tend to be more costly to purchase than inorganic N sources,
but many local or on-farm N sources may also be available. Some locally
available N sources may contain low concentrations of N, requiring
transportation and handling of large volumes of material. Cover crops are
useful, but may be problematic to fit into a specific cropping system,
depending on the length of growing season and rotational practices. As our
understanding of soil N and organic matter improves, better N management will
benefit all crop producers and the environment.

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About Me

I am a soil scientist with interest in managing plant nutrients in the best way possible. I am fortunate to be able to work in research and education to be able to accomplish this goal.
After receiving a PhD in Soil Science at the University of California (Riverside), I worked as a Research Scientist for the U.S. government, as a Professor of Soil Science, and now I work for a not-for-profit institution. It's been a wonderful experience!